1. Field of the Invention
The present invention relates to an image forming apparatus which is used as, e.g. a display panel. More particularly, the invention relates to the image forming apparatus in which a spacer is sandwiched along wiring between a first substrate, which has plural electron-emitting devices and the wiring for driving the electron-emitting devices, and a second substrate, which has an electrode defined at a voltage higher than that of the wiring.
2. Related Background Art
Generally, in the image forming apparatus in which the first substrate on the electron source side and the second substrate on the display surface side are arranged in an opposite manner while separated from each other, the spacer made of an insulating and the second substrate in order to withstand atmospheric pressure. However, the spacer is charged to affect an electron beam orbit near the spacer, which generates a problem in that light-emission position is shifted. This causes the image deterioration such as a decrease in luminance and bleeding in pixels near the spacer.
Conventionally, in order to prevent the charge of the spacer, it is known that the spacer coated with a high resistance film is used.
Specifically, it is known that the rib-shaped spacer coated with the high resistance film is sandwiched along the first-substrate wiring so that the high resistance film is electrically connected to the first-substrate wiring and the second-substrate electrode, or it is known that a spacer electrode is provided above and below the spacer coated with the high resistance film and the high resistance film is sandwiched between the wiring and the electrode through the spacer electrode so as to be electrically connected (for example, see Japanese Patent Application Laid-Open No. H8-180821 (U.S. Pat. No. 5,760,538)).
It is also proposed that intermediate layers (spacer electrode) having the electrical conductivity are provided on side faces on the first substrate side and second substrate side of the spacer coated with the high resistance film respectively and the electron beam orbit is controlled by the intermediate layer (spacer electrode) (for example, see Japanese Patent Application Laid-Open No. H10-334834 (U.S. Pat. No. 6,184,618)).
However, according to the inventors' study, for the image forming apparatus described in Japanese Patent Application Laid-Open No. H8-180821 in which the high resistance film is electrically connected to the first-substrate wiring and the second-substrate electrode without arranging the spacer electrode, it is newly found that sometimes the charge of the spacer is not sufficiently eliminated or sometimes the potential distribution of the spacer surface exhibits the unintentional distribution state.
Since the above-described phenomena depend largely on a process of manufacturing the display apparatus, the cause of the phenomena is not generalized. For example, the unpredictable distortion is generated between the first-substrate wiring and the second-substrate electrode, a foreign matter exists on the first-substrate wiring and the second-substrate electrode, and an unintentional burr is generated in the wring or the electrode. Therefore, the contact is not continuously achieved between the high resistance film of the spacer and the wiring or the electrode, and the position in which the high resistance film of the spacer is not partially in contact with the wiring or the electrode is generated, which causes the insufficient electrical contact. Particularly, in the wiring produced by the inexpensive manufacturing method, sometimes there is the partial difference in the surface shape, and the electrical contact failure is easy to be generated.
In the above case, not only the charge of the spacer is not sufficiently eliminated, but also the irregular change is generated in the potential distribution of the spacer surface, which results in the problem that the electron beam orbit does not corresponds to the design. Further, since the electron beam is accelerated from the first substrate to the second substrate, the orbit change emerges more remarkably by deflection force on the first substrate side compared with the second substrate side.
Referring to FIG. 10, the electron beam deflection caused by the potential distribution of the spacer surface on the first substrate side will specifically be described.
FIG. 10A shows the potential distribution of the surface of a spacer 3 when the high resistance film comes unintentionally into partial contact with wiring 5 of the first substrate in ranging the thin-plate-shaped spacer 3 coated with the high resistance film along the wiring 5. FIG. 10B is an equivalent circuit view of FIG. 10A. The numeral 11 in the drawing designates a second-substrate electrode, and the numeral 17 designates an equipotential line.
As shown in FIGS. 10A and 10B, assuming that resistance between a point C and a point A is R1, the corresponding resistance between a point D and the point B becomes R1 at the point B which is of a non-contacting area, the potential at the point B is increased by voltage drop generated by resistance R2 between the point A and the point B, when compared with the point A. Therefore, the orbit of the electron beam emitted from the electron-emitting device near the point B exhibits behavior different from the orbit of the electron emitted from the electron-emitting device near the point A, which results in the difference in image (distortion) between the point A and the point B.
On the other hand, for the image forming apparatus also described in Japanese Patent Application Laid-Open No. 8-180821 and Japanese Patent Application Laid-Open No. 10-334834 in which the spacer electrode is provided above and below the spacer coated with the high resistance film and the high resistance film is connected to the first-substrate wiring and the second-substrate electrode through the spacer electrode, an electric field distribution is generated near an area where the spacer electrode is exposed to the side face of the spacer. Although the electric field is substantially even in a lengthwise direction of the spacer (direction parallel to the wiring), the electric field emerges strongly when compared with the case in which the spacer electrode is not exposed. Therefore, an arrival position of the electron beam radiated from the adjacent electron-emitting device is easily largely disturbed due to misalignment in arranging the spacer. It is also found that the exposure of the spacer electrode to the side face of the space causes the discharge to largely decrease image quality. In order to prevent the large decrease in image quality, it is necessary that the spacer electrode is not exposed to the side face of the spacer, or it is necessary that the spacer is arranged with high accuracy, which causes the cost increase.